This application claims the priority of German application 199 22 861.2, filed in Germany on May 19, 1999, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a process for condensing a drafted fiber strand in a condensing zone arranged downstream of a front roller pair of a drafting apparatus, in which process the fiber strand is transported through the condensing zone by means of a perforated, suctioned transport means.
The present invention relates further to an apparatus for condensing a drafted fiber strand in a condensing zone arranged downstream of a front roller pair of a drafting unit, said apparatus comprising a stationary sliding surface, which has a suction slit extending essentially in the direction of the motion of the fiber strand, a perforated transport belt located between the fiber strand and the sliding surface, which transport belt transports the fiber strand over the sliding surface, the speed of said transport belt corresponding to at least the peripheral speed of the front roller pair, and a nipping roller bordering the condensing zone on its exit side.
A process and an apparatus of this type is prior art in U.S. Pat. No. 5,600,872. The condensing zone is bordered on its entry side as well as on its exit side by a nipping point, namely on the entry side by the nipping point of the front roller pair and on its exit side by a nipping point defined by the nipping roller, whereby the nipping roller presses against a driven bottom roller. The distance between the two nipping points is given in the known publication as 1.5 times that of the fiber length. A light tension draft is provided in the condensing zone, that is the nipping roller has a somewhat higher circumferential speed than the front roller pair. On reaching the condensing zone, the direction of motion of the fiber strand differs from the direction of motion of the transport belt.
A similar apparatus is prior art in German published patent application 197 22 528. According to this publication, the suction zone of the condensing zone is limited to between 10 and 25 mm, in order that, in the case of reduced suction performance, an improved condensing effect is attained. However, the distance between the nipping points is significantly larger than the suction zone and also larger than the fiber length. In the case of the latter publication, the matter of the airways of the transport belt becoming clogged with time is addressed. As a remedy, the suggestion is made to design the perforations of the transport belt as edge-free.
In the case of the known apparatus there is the risk that despite everything, fiber fly and trash can collect between the stationary sliding surface and the moving supporting surface of the transport belt. This trash build-up comes about, for example, in that individual fibers of the fiber strand to be condensed, in particular the shorter fibers, can insert themselves inwards with their front ends in the area of the suction slit in through the perforations of the transport belt. These fibers project then partly into the suction slit, namely until the suction slit has been passed. In the most favorable case, the relevant fiber can be cut off at the end of the suction slit and the cut-off end is suctioned off. In the least favorable case, the fiber end is simply turned over, so that it is not suctioned off, but rather is transported further with the transport belt, thus lying partly between the supporting surface and the sliding surface. This fiber can tear at any time, and can settle, in the form of dust, anywhere on the supporting surface or on the sliding surface, in particular in the case of sticky fibers containing honeydew. This results in time in varying condensing effects from spinning station to spinning station, so that at the exit side nipping points, varyingly condensed fiber strands are delivered.
The condensing of a drafted fiber strand, namely in an area to which the spinning twist is not yet retroactive, serves the purpose of bundling the fiber strand in its cross section and to make it less hairy overall. This results, after the spinning twist has been imparted, in a smoother yarn with a higher tear resistance. When, however, the desired condensing effect does not occur at even one spinning station due to an impaired condensing zone, a so-called Moire effect occurs in the later fabric, which can render it a reject product. It must be ensured, therefore, that the condensing effect at the individual condensing zones of the spinning stations is constantly maintained.
It is an object of the present invention to ensure that the perforations of the transport belt do not become clogged, and that in the area between the supporting surface of the transport belt and the stationary sliding surface no small deposits can form.
This object has been achieved in the process in accordance with the present invention in that the fiber strand has the same direction of motion as the transporting means when it reaches the condensing zone, and in that a sufficient amount of fibers located in the fiber strand continues to be nipped by the front roller pair during condensing.
The transport means has thus in the case of long fibers in the condensing zone--in the same direction--the same or a greater speed than the front ends of the fibers, which thus cannot insert themselves in the first place into the perforations of the transporting means. In the case of short fibers, however, it is sufficient when as high a percentage as possible of fibers are still controlled by the front roller pair as soon as the front ends are seized by the suction.
This object has been achieved in relation to the apparatus in accordance with the present invention in that the distance between the nipping points defined by the front roller pair on the one side and by the nipping roller on the other side is, in any case, not very much greater than the average staple length of the fibers located in the fiber strand.
This reliably prevents the fibers in the condensing zone from being seized by the suction force at their fronts ends and crawling forwards on the transport belt. The suction of the front end of the fibers into the perforations is thus effectively prevented.
In the case of short stapled material to be spun, the distance between the nipping points measures advantageously between 20 and 27 mm. The smaller value is selected for synthetic fiber material, in particular for viscose fibers.
In an embodiment of the present invention, the suction slit arranged directed against the transport belt has a shaft-like opening. Experience has shown that in the case of such openings of suction tubes in general, almost no fibers collect there. The shaft-like opening may taper in suction direction, which results in an acceleration of the suctioned fibers. In contrast, in the case of a suction slit in a metal sheet, in which the height of the suction slit corresponds to the thickness of the metal sheet, the risk of clogging, caused by increased turbulences, would arise.
The shaft-like opening is formed for the purpose by an exchangeable fitted window. The suction slit can then be adapted easily to various fiber materials.
In a further embodiment of the present invention, the front roller pair can have a bottom roller which is provided on its periphery with a profiling, which is disposed with a light pressure on the transport belt outside of the condensing zone.